A fuel assembly in a boiling water nuclear reactor comprises a long tubular container. The container is often made with a rectangular or square cross section and is open at both ends to make possible a continuous flow of coolant through the fuel assembly. The fuel assembly comprises a large number of equally long tubular fuel rods, arranged in parallel in a certain definite, normally symmetrical pattern. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To allow coolant to flow along the fuel rods in the desired way, it is important that these be spaced from each other and prevented from bending or vibrating when the reactor is in operation. For this purpose, a plurality of spacers are used, distributed along the fuel assembly in the longitudinal direction.
Experience has shown that in connection with repair and service of nuclear reactors, foreign matter may enter the coolant of the nuclear reactor. The foreign matter then moves with the coolant which circulates through the reactor core. The foreign matter may, among other things, consist of metal chips formed in connection with repair of, for example, a steam separator, pieces of metal wire or oxide flakes which have been torn loose from, for example, the fuel rods where they have been formed. The foreign matter may give rise to wear damage which may result in serious consequences if the damage arises on parts which are particularly fragile, such as on the fuel rods.
To avoid damage of the above kind, it is known to design various kinds of filters close to the lower part of the fuel assembly. The filter prevents the foreign matter from reaching the fuel rods. It is known, for example, to design the bottom tie plate, to which the fuel rods are attached, with a large number of through-holes. In this way, a strainer is provided, which prevents foreign matter from reaching the fuel rod bundle with control-rod guide tubes and spacers. It is also known to arrange various types of helical springs in a so-called bottom support in which the fuel bundle is arranged to rest. The helical springs are adapted to prevent debris from passing with the coolant flow through the bottom support and up through the bottom tie plate.
By arranging additional structural parts in the flow path of the coolant through the fuel assembly, the pressure drop across the fuel assembly increases. The pressure drop may be increased within certain limits. Too high a pressure drop may, in the worst case, lead to so-called dryout. To minimize the risk of dryout, the fuel assembly is formed with a flow of coolant which with a fixed margin, the so-called dryout margin, exceeds the coolant flow where dryout occurs under the present conditions.
A disadvantage of arranging loose parts in the bottom tie plate or in the bottom support for capturing any foreign matter is that these may be set into vibration because of the high pressure of the coolant. During the vibration, wear arises, whereby the loose parts may become detached. It may also result in flakes of the material, against which the wear occurs, loosening and accompanying the coolant.